The overall purpose of this proposal is to develop and test a brachytherapy dose calculation algorithm that will accurately and efficiently predict absorbed dose in the presence of geometrically complex applicator and tissue heterogeneities. The scope of this work includes both medium energy (192Ir, 137Cs) and low energy (125I) photon-emitting radionuclides. The intent is to develop a general, but practical algorithm, analogous to heterogeneity corrections used in external beam dosimetry, which can be integrated into a clinical computerized treatment planning system. Because no such computational method currently exists, dose calculations ignore applicators such as gynecological colpostats or variations in the composition of the implanted tissue even though experimental evidence indicates dosimetry errors as large as 50% may result. The great improvement in dosimetric accuracy expected as a result of this work will be of significant value in formulating definitive normal tissue dose-response curves in intracavitary therapy, and in dosimetrically optimizing the design of applicators in all areas of brachytherapy so as to minimize normal tissue complications while maximizing tumor control. Monte Carlo simulation will be the principal tool for predicting definitive dose perturbation factors in well-defined heterogeneous implant geometries. In addition to characterizing the magnitude of dose perturbations for various sources, these data will be used to validate the practical heterogeneity correction algorithms described above. Because of the fundamental importance of Monte Carlo calculation as the primary source of dose estimates near interstitial sources, a limited, but carefully defined set of experimental measurements is proposed to verify the accuracy of this computational approach.